Energy storage and conversion apparatus

ABSTRACT

An energy storage and conversion apparatus (1) comprising a containment (5) defining a vacuum chamber (7), a substantially vertical shaft within the vacuum chamber (7), a stator (11) on the shaft (9), and a cylindrical rotor (13) which, in use, is driven by the stator (11) to store energy as kinetic energy of the rotor (13) and acts with the stator (11) as a generator to release energy, wherein the rotor (13) is supported by the shaft (9) via an end cap (29), positioned at the upper end of the cylindrical rotor (13), which engages the shaft (9). The rotor (13) is suspended from a pin bearing (31) which is attached to an end cap (29). The pin bearing (31) sits in a bearing cup (37) mounted in a damper (39) at the end of the shaft (9). A magnet bearing (43) is provided at the lower end of the rotor (13) to assist in positioning the rotor (13) relative to the stator (11).

This is a divisional of application Ser. No. 08/766,768, filed Feb. 28,1997, now pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to energy storage and conversion apparatus, andin particular to an apparatus wherein a cylindrical rotor is driven by astator within the rotor to store energy as kinetic energy of the rotorand wherein energy can be withdrawn from the rotor when the stator androtor act as a generator.

2. Discussion of Prior Art

Energy storage and conversion apparatus of the aforementioned type havealready been described in some of the present applicant's earlier patentspecifications. The applicant has, however, continued to develop itsenergy storage and conversion apparatus and, as a result thereof, hasdesigned an apparatus according to the present invention.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is providedan energy storage and conversion apparatus comprising

a containment defining a vacuum chamber,

a substantially vertical shaft within the vacuum chamber,

a stator on the shaft, and

a cylindrical rotor which, in use, is driven by the stator to storeenergy as kinetic energy of the rotor and acts with the stator as agenerator to release energy,

wherein the rotor is supported by the shaft via an end cap, positionedat the upper end of the cylindrical rotor, which engages the shaft.

By suspending the rotor from the shaft about the stator, a very neat,compact and reliable energy storage and conversion apparatus results.

In a preferred embodiment, the end cap includes a central pin bearingacting on the upper end of the shaft. The pin bearing preferablyincludes a substantially spherical head.

More preferably, the pin bearing is a spherical spiral groovehydrodynamic pin bearing. The pin bearing may be formed from steel, thespiral groove being etched into the surface of the spherical head.

By using a spherical spiral groove pin bearing, high axial loads can beaccommodated with only very small friction losses. In contrast, normalprior art energy storage and conversion apparatus use eitherconventional roller bearings, which result in high friction losses, orelectro-magnetic bearings which are complicated, costly and potentiallyunreliable.

The pin bearing is preferably received in a bearing cup mounted in adamper at the end of the shaft. More preferably the damper includes oilwhich also acts to lubricate the pin bearing.

The end cap may be formed from a composite material, such as a carbonfibre composite, aluminium, maraging steel or any other appropriatematerial.

In one embodiment, the end cap is received in the upper end of thecylindrical rotor by means of a friction fit. Alternatively, the end capmay be physically joined to the cylindrical rotor by any appropriatemeans.

A magnet bearing is preferably provide towards the lower end of therotor to assist in positioning the rotor relative to the stator. Themagnet bearing is preferably a permanent magnet bearing acting betweenthe shaft and the rotor. Alternatively, the magnet bearing may be anelectro-magnet bearing.

If a permanent magnet bearing is used, annular rings of north and southpoles are preferably provided on the inside surface of the rotor and onthe shaft such that the opposing poles repel. If such an arrangement isused, the rotor is held out of contact with the stator and may, if thearrangement, number and position of the poles is chosen carefully,assist in lifting the rotor slightly to reduce the pressure of the pinbearing.

In another embodiment, a magnet bearing may act on the lower end of thecylindrical rotor from below to assist in lifting the rotor.

According to a second aspect of the present invention, there is providedan energy storage and conversion apparatus comprising

a base member,

a containment mounted on the base member defining a vacuum chamber,

a substantially vertical shaft within the vacuum chamber,

a stator on the shaft, and

a cylindrical rotor which, in use, is driven by the stator to storeenergy as kinetic energy of the rotor and acts with the stator as agenerator to release energy,

wherein the shaft is mounted to the base member such that, in the eventof a failure of the apparatus, the energy stored in the rotor ispreferentially transferred to the shaft rather than to the containment.

By providing an apparatus of this kind, torque forces resulting from acrashing rotor will not all impact themselves on the machine containmentas is the case in many prior art flywheel energy storage and conversionsystems. Instead the torque forces will be transmitted via the stator tothe central shaft and from the shaft either directly to the machine baseplate or be dissipated in a friction joint on the shaft (for examplebetween the motor/generator and the shaft). The containment maytherefore have a reduced wall thickness than is normally required towithstand such high energy dissipation.

Preferably the spacing between the rotor and the stator on the shaft issubstantially less than the spacing between the rotor and thecontainment. This arrangement ensures that, if a rotor does fail, itwill crash initially into the stator/shaft unit rather than thecontainment wall. Energy will, therefore, be transferred to the basemember immediately upon failing of the apparatus or be dissipated in thefriction joint on the shaft.

The base member may include a recess for receiving an end of the shaft,the shaft being received in the recess with a tight fit. Other ways ofengaging the shaft with the base member can, of course, alternatively beused (for example, a friction joint for energy dissipation).

Preferably the shaft is formed of high strength aluminium. Any othersuitable material could alternatively be used.

Preferably the shaft is hollow to accommodate a pin bearing forsupporting the rotor. The shaft may, however, simply have a recess atits upper end for receiving the pin bearing, rather than beingcompletely hollow.

The base member is preferably adapted to be attached to a support havingsignificant mass which safely disperses energy from the rotor, ifnecessary.

Although not essential for the implementation of the present invention,the length of the rotor is preferably at least twice the externaldiameter of the rotor. If this requirement is satisfied, however, a tallrelatively thin unit is provided which includes a significant length ofshaft per mass of rotor. Hence, a safer apparatus may result.Furthermore, by forming a tall, relatively thin unit, a larger number ofunits can be accommodated in any given floor area than is the case withthe known prior art apparatus.

According to a third aspect of the present invention, there is providedan energy storage and conversion apparatus comprising

a containment defining a vacuum chamber,

a substantially vertical shaft within the vacuum chamber,

a stator on the shaft, and

a cylindrical rotor which, in use, is driven by the stator to storeenergy as kinetic energy of the rotor and acts with the stator as agenerator to release energy,

wherein the rotor comprises an inner layer of glass fibre and an outerlayer of carbon fibre. More preferably, the inner layer is a glass fibrecomposite material and the outer layer is a carbon fibre compositematerial. In a preferred embodiment, the glass fibre composite isE-glass.

By not including any solid metallic components in the rotor, which is inmarked contrast to most of the prior art apparatus other than thepresent applicant's apparatus, there is less likelihood that a rotorfailure and consequent flying debris will result in rupture of theapparatus containment. Indeed, by using a glass/carbon fibre compositerotor and carbon fibre end cap design rotating closely about the centralshaft, the rotor may even remain essentially intact during a crashsituation.

Preferably the inner layer of the rotor contains a material magnetisedto form a multipolar magnetisation for interaction with the stator,during use. More particularly, the inner layer preferably includes anannulus of alternating north and south poles which enable the stator,having a number of poles produced by the stator core with windingsthereon, to drive the rotor to store energy.

Further, the inner layer of the rotor may contain a material magnetisedto form at least one homo-polar radial magnetisation for interactionwith a magnet mounted on the shaft to produce a bearing for the rotor.As indicated above, a number of homo-polar radial magnetisations may beprovided, thereby resulting in an improved bearing providing a degree oflift to the rotor. The arrangement may also include an axial magnetthrust bearing which acts against the bottom face of the rotor, therebyfurther improving the degree of lift to the rotor.

The magnetised material is preferably a powder introduced into the innerlayer of the rotor during manufacture. The powder may be ferrite orNdFeB. Any other appropriate material could, of course, alternatively beused.

The inner and outer layers are preferably strain matched to preventseparation during use.

The thickness of the inner layer is preferably about two thirds of thethickness of the complete rotor. By producing a rotor having thesedimensions, the rotor has a significant amount of mass provided by theglass composite and yet is held together during spinning of the rotor byvirtue of the strong external layer of carbon fibre composite. With thisin mind, the rotor may spin at between approximately 1,200 Hz and 1,800Hz, for example.

The magnetized material preferably extends from the inner surface of theinner layer to about half way through the inner layer. The depth of themagnetised material can, of course, be altered to suit the requirementsof a particular rotor/stator configuration.

As mentioned above, an end cap of the rotor, preferably including a pinbearing, may assist in suspending the rotor on the shaft.

According to a fourth aspect of the present invention, there is providedan energy storage and conversion apparatus comprising

a plurality of stators,

a corresponding plurality of cylindrical rotors arranged to rotate aboutthe stators and

means for containing the stators and rotors,

the stators in use driving the rotors to store energy as kinetic energyof the rotors and interacting with the rotors to act as generators torelease energy,

wherein the containment means define a plurality of chambers within aunitary structure in which the stators and rotors are accommodated.

As far as the applicant is aware, a single structure accommodating aplurality of energy storage and conversion apparatus units has neverbefore been suggested.

The unitary structure preferably comprises a honeycomb-type structure inwhich a plurality of cylindrical chambers are regularly arranged.

Preferably each chamber accommodates a single stator and correspondingrotor. However, a single chamber may accommodate more than onestator/rotor unit in some circumstances.

The advantages of such a honeycomb arrangement in a single unit are thata greater number of rotors can be accommodated in the smallest areapossible since the rotors share common containment walls, and thosewalls internal to the structure do not need to be as thick as would berequired in a single machine since a breech from one rotor chamber toanother would not endanger personnel. Further, the common mass of aunitary structure of this size is sufficient to absorb the kineticenergy of a crashing rotor without any special bolting downarrangements.

The unitary structure may be fabricated to include any number of rotorsdepending on the total energy storage requirements of the application.The arrangement shown for illustration purposes contains 37 chambers.

The unitary structure may be formed from a plurality of extrusions cutto length the welded together. If the unitary structure is formed inthis way, preferably a minimum number of different shaped extrusions areused to produce the complete unitary structure. By way of example astructure is illustrated which employs only three different extrusiontypes.

Preferably, each chamber is closed by an end flange incorporating anon-return valve. Although each chamber may be provided with a separatevacuum pump, it is preferable that the unitary structure be encased in acommon vacuum chamber so that the common vacuum chamber can be pumpedout, thereby resulting in a vacuum being produced in each rotor chamber.In the event of a crash of any single rotor in the unitary structure,the instantaneous release of light gases from the rotor material andsubsequent pressure rise in the rotor chamber will cause the non-returnvalve to close thus isolating that chamber from the other chambers inthe structure and preserving the integrity of the remaining rotors.

Although the unitary structure may be manufactured from aluminium, anyother appropriate material can, of course, alternatively be used.

A common cooling system may be provided for all the stators and rotorsof the complete apparatus. This is clearly preferably to having separatecooling systems for each stator/rotor unit.

Preferably the containment means includes a getter for removing gas fromthe chambers to improve the vacuum. Silicon or carbon based getters arethe preferred choice for this.

Each stator and rotor unit can preferably store up to 20 kWhr, morepreferably about 5 kWhr, of energy.

Although not specifically stated to date, it should be appreciated thatany of the features of the various aspects of the present inventiondescribed herein may be combined with any other aspect to produce anenergy storage and conversion apparatus which is both novel andinventive over the known prior art.

Furthermore, in an energy storage and conversion apparatus according tothe present invention, an external circuit may be provided through whichgases in the containment are driven by a pressure difference, theexternal circuit including a device for removing gas, thereby improvingthe vacuum within the containment.

Moreover, the speed of a rotor may be measured to provide an outputindicative of the energy stored in the rotor. More preferably, a visualoutput is provided giving the energy available from the apparatus inreal time.

In a particular embodiment, the speed of the rotor may be measured bymonitoring the switching frequency of the motor/generator powerelectronics.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention are now described, by waysof example only, with reference to the accompanying drawings in which:

FIG. 1 is sectional side view of an energy storage and conversionapparatus according to the present invention;

FIG. 2 is a schematic cross-sectional side view of the inner layer of arotor, somewhat shortened, which could be used in an apparatus as shownin FIG. 1;

FIG. 3 is a view in the direction A--A of the rotor of FIG. 2;

FIG. 4 is the same view as in FIG. 3, but wherein windings of amagnetising fixture for magnetising the rotor are shown;

FIG. 5 is a plan view of a containment for accommodating a plurality ofstator/rotor units;

FIG. 6A is a schematic side view of a maintenance bell for a getterhousing assembly mounted on a side of an energy storage and conversionapparatus containment wall;

FIG. 6B is an enlarged side view of the getter housing assembly shown inthe maintenance bell of FIG. 6A; and

FIG. 7 is a schematic side view of an energy storage and conversionapparatus according to the present invention incorporating an externalcircuit for removing gas from the vacuum chamber of the apparatus.

DETAILED DISCUSSION OF PREFERRED EMBODIMENTS

With reference to FIG. 1, an energy storage and conversion apparatus 1comprises a base member 3, a containment 5 mounted on the base member 3defining a vacuum chamber 7, a substantially vertical shaft 9 within thevacuum chamber 7, a stator 11, mounted on the shaft 9 and a cylindricalrotor 13 which, in use, is driven by the stator 11 to store energy askinetic energy of the rotor 13 and acts with the stator 11 as agenerator to release energy. The electrical contacts to the stator 11(for energising the stator 11 to drive the rotor 13) are not shown inthe enclosed drawings, but may pass along the hollow bore 9a of theshaft 9.

the stator 11 is not shown in any detail in FIG. 1, but may be of anyappropriate type incorporating a core defining a plurality of poles,such as 4 poles, about which coils are wound to produce magnetic fluxwhich is directed by the pole faces towards the rotor 13 to cause therotor 13 to rotate. In this way, energy can be stored as kinetic energyof the rotor 13. Conversely, if energy is to be withdrawn from theapparatus 1, the rotor 13 and stator 11 can act as a generator orgenerator to produce an electrical output via the power electronics (notshown) of the apparatus.

The base member 3 of the apparatus 1 has significant strength by virtueof its thickness and the material from which it is made, which may bealuminium, for example. Holes 15 through the base member 3 are shown forreceiving bolts 17 for securing the base member 3 to a floor 19 or thelike of considerable mass and strength. As a result, the energy storageand conversion apparatus 1 will be held firmly in position, even if theapparatus 1 fails.

In the event of a failure of the apparatus 1, the energy stored in therotor 13 is prevented from destroying the containment 5 by virtue of theshaft 9 being solidly mounted to the base member 3. More particularly,the lower end 21 of the shaft 9 is received in a recess 23 in the basemember 3 with a tight fit. Means (not shown) for strengthening the jointbetween the shaft 9 and the base member 3 can also be used. Further, theshaft 9 is made of a high strength material, such as aluminum, so thattorque forces and energy imparted by the rotor 13 during a failure ofthe apparatus 1 will be transferred to the base member 3, and hence thesolid support 19, via the shaft 9.

It should also be noted that the rotor 13 has a length which is at leasttwice its external diameter so that a tall, relatively thin apparatus 1results. This arrangement also means that there is a significant lengthof shaft 9 for absorbing torque forces and energy from the rotor 13 inthe event of a failure of the apparatus 1. A safer apparatus 1 is,therefore, provided and the containment 5 does not need to have aparticularly large wall thickness. In practice, of course, thecontainment 5 would be designed to provide significant shielding againsta rotor failure.

As can be seen from FIG. 1, the rotor 13 is formed with an inner layer25 of E-glass and an outer layer 27 of carbon fibre composite. Othersuitable materials could, however, alternatively be used, provided thatthey provide the required properties for the rotor. In this regard, theinner layer 25 of E-glass is relatively cheap and provides a reasonableamount of mass to the rotor 13. The E-glass is also able to receivemagnetisable material, in the form of particles or powder, between thefibres or tows of the glass fibre in the E-glass. As can be seen fromFIG. 2 which only shows the inner layer 25 of the rotor 13, themagnetisable material is preferably only entered into the inner half 25aof the inner layer 25 of the rotor 13. The outer layer 27 of the rotor13 is included primarily to support the inner layer 25 and is,therefore, formed of a material having significant strength whenspinning at high speed, such as 1,200-1,800 Hz. Carbon fibre compositesare particularly suitable for this.

The rotor 13 includes an end cap 29 made of maraging steel, aluminum orcarbon fibre composite which mounts a pin bearing 31 as shown in FIG. 1.The pin bearing comprises a shaft 33 carrying a spherical ball 35 at itsfree end. The spherical ball 35 is etched, during manufacture, such thatspiral grooves are formed in the surface thereof. The spherical ball 35,or head, of the pin bearing 31 is received in a cup 37 mounted in adamper 39 positioned at the end of the shaft 9. The damper 39 extendsinto the bore 9a of the shaft and is retained therein by means of sideflanges 41 abutting the upper end of the shaft 9. The damper 39 carriesoil which acts to dampen the radial and axial motion of the cup 37 asthe rotor 13 moves, thereby resulting in damping of the complete energystorage and conversion apparatus 1. The oil in the damper 39 also actsas a lubricant for the pin bearing 31 between the head 35 of the bearing31 and the surface of the cup 37. As will be appreciated, as the rotor13 spins, the spiral grooves in the head of the pin bearing 31 drive oilbetween the head 35 and the cup 37 to lift slightly the rotor 13 onto afilm of oil. The rotor 13 is, therefore, free to spin with negligiblefriction, resulting in minimal energy being lost through the bearing.This is clearly desirable.

At the lower end of the rotor 13 a permanent magnet bearing 43 isprovided to ensure, in combination with the pin bearing 31, that therotor 13 does not clash with the stator 11. More particularly, apermanent magnet 45 is mounted on the shaft 9 with, in this case, anorth pole of the magnet 45 facing the inside surface of the rotor 13.As can be seen in FIG. 2, the magnetisable material within the innerlayer 25 of the rotor 13 is magnetised with a north pole 47 and a southpole 49 formed annularly. The north pole of the magnet 45 and the northpole 47 of the rotor 13 face each other and hence provide a repellingforces and the south pole 49 of the rotor 13 is attracted towards thenorth pole of the magnet 45. By virtue of this arrangement, the rotor 13is kept clear from the stator 11 and the rotor 13 is provided with alittle lift to assist in reducing friction between the pin bearing 31and the cup 37.

An additional axial bearing may also be provided comprising anotherpermanent magnet 51 (see FIG. 1) for acting in conjunction with amagnetised region on the end of the rotor 13 to repel the rotor 13 andthereby lift the rotor 13.

With reference to FIGS. 2-4, an inner layer 25 of the rotor 13 whichcould be used in an energy storage and conversion apparatus according tothe present invention is shown. In FIG. 2, however, the rotor 13 isshown significantly shortened. As can be seen, a radial multi-polarmagnetisation 51, to enable the rotor to act as a motor/generator, isshown in the central region of the rotor 13. At the lower end of therotor 13, homo-polar radial magnetisation is shown which can interactwith a permanent magnet (as described above) or an electromagnet mountedon the shaft 9 to assist in suspension of the rotor 13 about the stator11. Although only one north pole 47 and and one south pole 49 are shownon of the rotor 13, additional poles and additional permanentmagnets/electromagnets could be utilised to strengthen the interactionbetween the rotor 13 and the magnets/electro magnets mounted on theshaft 9, depending upon the forces required.

The magnetised regions 47, 49, 51 of the rotor 13 are produced by actingon virgin magnet material included in the inner half 25a of the innerlayer 25 of the rotor 13 during manufacture of the rotor 13. Although itis possible to introduce pre-magnetised material into the rotor 13 andto align the material as required during manufacture of the rotor 13, arotor 13 as described herein preferably has the magnetisation applied tothe rotor 13 after the composite materials of the rotor 13 have cured.This is achieved by impressing on the virgin magnet material within therotor 13 a magnetisation using a fixture which consists of a series ofcoils 53 which, when excited, produce a magnetic field of the formrequired in the magnet (see FIG. 4). The field required to magnetise themagnetic material, which may be ferrite, NdFeB or any other appropriatematerial, depends on the material type. For example, 1.5 Tesla isrequired for ferrite, whereas 4 Tesla is required for NdFeB. The fieldis produced by a single, high current pulse from a capacitor dischargeunit, which current may be in the region of 30,000 amps. Once the fieldhas been applied to the rotor 13, the fixture is removed leaving thepermanent magnetisation as shown in FIGS. 2 and 3, for example. As willbe appreciated, it is simply necessary to design a fixture for aparticular application to achieve a desired magnetisation in the rotor13.

Although to date an energy storage and conversion apparatus 1 has beendescribed which incorporate a single stator 11 with a single rotor 13,the present invention further provides an energy storage and conversionapparatus comprising a plurality of stators 11, a correspondingplurality of cylindrical rotors 13 arranged to rotate about the stators11 and containment means 100 defining a plurality of chambers 102 withina unitary structure in which the stators 11 and rotors 13 areaccommodated. Such a containment 100 is shown in FIG. 5.

The energy storage and conversion apparatus shown in FIG. 5 is extremelyneat and compact by virtue of the arrangement of cylindrical chambers102 in the unitary structure. As a result, an apparatus having the powerstorage and conversion capability of 37 apparatus as shown in FIG. 1 isprovided without requiring an unreasonable amount of space. As will beseen from FIG. 5, adjacent vacuum chambers 102 share common containmentwalls 104.

The honeycomb-type structure shown in FIG. 5 is formed from threedifferent shaped extrusions 106, 108, 110 (cf. the shaded area in FIG.5). The extrusions are made of aluminium or any other appropriatematerial and are simply cut to length. Adjacent extrusions are thenwelded together by weld joints 112, as shown in FIG. 5. It will beunderstood however, that the choice of extrusion shape, and number ofdifferent shapes used to fabricate the unitary structure are basedpurely on commercial reasons to minimise fabrication costs and thechoice of extrusion shapes and numbers of different shapes used does notaffect the validity of the final structure.

Although in theory each chamber 102 could have an end flange at eitherend and a separate means for preserving the vacuum in the chamber 102,an apparatus according to the present invention ideally has either anexternal casing (not shown) around the complete unitary structure or anend capping covering at least one end of the unitary structure. Ineither event, each chamber 102 should still have its own end flanges(not shown); a non-return valve may then be provided in each end capprotected by the outer capping or casing. In such as arrangement, asingle vacuum pumping device can be provided for the complete unitarystructure, gas within the individual chambers 102 being drawn outthrough the non-return valves to be removed by the main vacuum pump.Further, if an individual stator 11/rotor 13 unit fails, resulting inmolecules being released to the rotor chamber, the remaining units willbe affected due to the protection provided by the closing of thenon-return valve.

Moving on now to FIGS. 6A and 6B, a getter housing assembly 61 andmaintenance bell 63 are shown. The getter housing assembly 61 provides amount for a getter material 65, such as silica gel, activated charcoal(possibly in the form of a cloth or fabric formed by pyrolysis), whichabsorbs gas molecules to improve the vacuum within the vacuum chambers7, 102. In this regard, as will be appreciated, the better the vacuumwithin the vacuum chamber 7, 102, the less friction will result and,accordingly, less energy will be lost from the rotor 13. Hence, a highervacuum is essential for successful running of an energy storage andconversion apparatus according to the present invention.

With specific reference to FIG. 6B, the getter material 65 is mounted onan end cap 67 of the greater housing assembly 61 such that the gettermaterial 65 is positioned adjacent to the rotor 13. A cylindrical wall69 of the getter housing 61 is attached to the containment 5 of anenergy storage and conversion apparatus 1. Seals 71 are provided betweenthe greater housing 61 and the containment 5, and between the end cap 67and the cylindrical wall 69 of the housing 61. To enable the greater 65to be serviced or replaced, the maintenance bell 63 (FIG. 6A) is used.This incorporates a wall 73 for encasing the getter housing 61, anaccess 75 to a vacuum pump for producing a high vacuum within themaintenance bell 63 and robotic or other maintenance tools 77 forinteracting with the getter housing 61. Hence, when the maintenance bell63 has been attached to the containment 5 and a vacuum has been producedwithin the maintenance bell 63, the tools 77 can be used to remove theend cap 67 and attached getter material 65 from the getter housing 61.Replacements of the getter material 65 can then be achieved withoutspoiling the vacuum around the rotor 13 within the vacuum chambers 7,102 of the energy storage and conversion apparatus 1. Once replacementof the getter material 65 has been achieved, the end cap 67 of thegetter housing 61 is replaced prior to the maintenance bell 63 beingremoved.

Another form of apparatus for removing gases present in the vacuumchamber 7 of an energy storage and conversion apparatus 1 according tothe present invention is shown in FIG. 7. In this Figure, a molecularpump 79, comprising a plurality of helical grooves 81, faces the outsideof the rotor 13. As the rotor 13 rotates, gases produced by off-gasingfrom the rotor, or other gases within the containment 5, are driven bythe molecular pump 79 upwards in FIG. 7. This results in a low pressureregion being formed towards the bottom of the vacuum chamber 7 and ahigh pressure region being formed towards the top of the vacuum chamber7.

An external pipe circuit 83 is shown incorporating a gas remover device85, which may be an ionisation pump or a getter material. Hence, due tothe pressure differential between the high pressure region and the lowpressure region in the vacuum chamber 7, gas is driven through theremover device 85 and is thereby removed from the system. An improvedvacuum can therefore be achieved within the vacuum chamber 7.

To assist in servicing of the remover device 85, valves 87 are providedon either side of the remover device 85. When these valves 87 areclosed, the remover device 85 can be disconnected from the pipe circuit83 for servicing. The remover device 85 then simply needs to bereinstated into the circuit 83 and that part of circuit 83 between thevalves 87 needs to be pumped out to produce a vacuum prior to the valves87 being reopened. Hence, a very simple and user friendly arrangement isprovided for improving the vacuum within the vacuum chamber 7 of theenergy storage and conversion apparatus 1.

Although not shown in the drawings, the speed of the rotor 13 can bemeasured, such as by monitoring the switching frequency of themotor/generator power electronics, to provide an output indicative ofthe energy stored in the rotor 13 at any particular time. Morepreferably, a visual output is provided giving the energy available fromthe energy storage and conversion apparatus 1 in real time.

It will of course be understood that the present invention has beendescribed above purely by way of example, and that modifications ofdetail can be made within the scope of the invention.

What is claimed is:
 1. An energy storage and conversion apparatuscomprisinga base member, a containment mounted on the base memberdefining a vacuum chamber, a substantially vertical cantilever shaftwithin the vacuum chamber, a stator on the shaft and a cylindrical rotorwhich, in use, is driven by the stator to store energy as a generator torelease energy, wherein the shaft is cantilever mounted to the basemember such that, in the event of a failure of the apparatus, the energystored in the rotor is preferentially transferred to the shaft ratherthan to the containment.
 2. An apparatus as claimed in claim 1, whereinthe spacing between the rotor and the stator on the shaft issubstantially less than the spacing between the rotor and thecontainment.
 3. An apparatus as claimed in claim 1, wherein the basemember includes a recess for receiving an end of the shaft, the shaftbeing received in the recess with a tight fit.
 4. An apparatus asclaimed in claim 4, wherein the shaft is formed of high strengthaluminium.
 5. An apparatus as claimed in claim 4, wherein the shaft ishollow to accommodate a pin bearing and damper for supporting the rotor.6. An apparatus as claimed in claim 1, wherein the base member isadapted to be attached to a support of significant mass which can safelydisperse energy from the rotor, if necessary.
 7. An apparatus as claimedin claim 1, wherein the length of the rotor is at least twice theexternal diameter of the rotor.
 8. An apparatus as claimed in claim 1wherein an external circuit is provided through which gases in thecontainment are driven by a pressure difference, the external circuitincluding a device for removing gas thereby improving the vacuum withinthe containment.